it works with or without “gui”:
format slot of player
format gui slot of player
@Nd_ at the end of the condition, it should be this:
“to close then run” not “to close and run” (not sure if this will fix it)

Use any of these:
format slot 1 of player with splash potion of harming named "{name}" with lore "{lore}" to close
format slot 1 of player with splash potion of harming named "{name}" with lore "{lore}" to close and run "command"
format slot 1 of player with splash potion of harming named "{name}" with lore "{lore}" to be unstealable
format slot 1 of player with splash potion of harming named "{name}" with lore "{lore}" to close and run [make player execute "command"]
format slot 1 of player with splash potion of harming named "{name}" with lore "{lore}" to close and run [make console execute "command"]

Not too sure if this (json.sk link)is compatible with Minehut 1.14.4 but here we go:
It allows Skripters to implement clickable messages. (this is why I want this addon)
EXAMPLE: json("%player%", "&7&oHello, this is a &bsample json&7.||ttp:&bI'm a tooltip for the first cluster.|| There's no tag, so I've started a new cluster.||cmd:/ping||ttp:&6&o&lCLICK&f for /ping")
Downside is that it's long... But it's worth it.
Hopefully this is compatible. Thanks for your time.